1. Field of the Invention
The present invention relates to optical transmission of data and, in particular, an optical transmission controller for controlling the optical transmission of data.
2. Discussion of Related Art
In general, optical transmission of data requires a laser, optical modulators, and coupling of light into an optical fiber. Therefore, parameters associated with the stability and output power of the laser, the optical modulators, and the total output power of the optical transmitter need to be controlled. Currently, developers of optical data transmission equipment assemble and debug equipment with modular components from various vendors in order to perform these tasks. Integrating these various components into a complete and operational transmission system is often a difficult and time-intensive undertaking.
At present, no integrated solution exists for general optical applications in which it is required to drive a laser, control the optical modulators according to a modulation scheme, and control the overall output power of the optical transmission system. As presently practiced, each of these functions may be performed by an individual integrated circuit solution. For example, the Hytek temperature control module, manufactured by Hytek Microsystems of Carson City, Nev., is designed to stabilize the operating temperature of a laser diode device. Further, Pacific Wave Communications Mach-Zehnder modulators, manufactured by Pacific Wave Industries, Los Angeles, Calif., provide control of the modulators. Each device can be integrated into an overall control system, which typically requires a substantial investment of engineering resources. For example, each device must be controlled by a software control program, and therefore requires interfacing with a computerized control system. The software control process is complicated when each component is controlled by different, and often incompatible, interface protocols and command sets.
Additionally, in optical transmission systems with electro-opto signal modulation devices such as lithium niobate (LiNbO3) Mach-Zehnder modulators, DC bias voltage stabilization is very important. The various bias control methodologies discussed (See, e.g., U.S. Pat. Nos. 5,003,624; 5,453,608; and 5,726,794), have important limitations.
One limitation with previously proposed methodologies is in bias drift detection. Bias drift detection is based on time-domain waveform analysis of the signal strength at the modulator output in response to a pilot tone or dither tone modulation applied to the modulator DC bias port. Very low waveform distortion and large pilot tone amplitude are required for good control accuracy, which in time can cause large interference in the modulator output beams.
Another limitation is that the pilot tone applied to the modulator DC bias port and detection circuit relies on changes in the average optical power output by the modulator measured in response to the dither tone modulation. This method does not work when a large RF signal is present at the modulator RF port because the average power remains relatively constant with little change. As an example, an NRZ data signal with an amplitude equal to the Vπ of the modulator, as is the case in many such data transmission systems, has a constant average optical signal output power that is independent of any particular bias voltage.
An additional limitation is that these methods can only stabilize the bias voltage for a single modulator and in some cases only at the linear operating point. However, there is a need to bias modulators at points other than the linear operating point. Additionally, transmission systems typically include more than one modulator in order to implement complex data-encoding schemes.
Therefore a need exists for the integration of the various functions of the optical control circuitry onto a single chip or board. There further is a need to interface the optical control circuitry in a convenient way with an outside environment, such as, for example, a computer system. Further, there is a need for optical control circuitry that can operate with multiple modulators and various RF signal inputs in order to support complex data transmission systems.